1. Field of the Invention
The present invention relates to the field of non-volatile memorys, and more particularly, to a contactless channel write/erase flash memory cell/array and method of fabricating the same.
2. Description of the Prior Art
FIG. 1 is a cross-sectional view illustrating a conventional flash memory cell 10. FIG. 2 is a cross-section view illustrating a metal contact structure associated with the conventional flash memory cell structure. Referring to FIG. 1, the flash memory cell 10 is built upon a P-substrate 11 including a N-well 12 formed on the P-substrate 11 and a stacked gate 14 formed on the N-well 12. An N+-doped region 16 and an N+-doped region 18, functioning as a source and a drain of the flash memory cell 10, respectively, are formed two sides of the stacked gate 14 in the N-well 12 respectively. A P-doped region 20 is formed surrounding the N+-doped region 18 in the N-well 12 and a P-doped region 22 is formed beneath the stacked gate 14.
The stacked gate 14 includes a control gate 24 and a floating gate 26. A word line voltage VWL is applied to the control gate 24 for controlling the flash memory cell 10. The floating gate 26 is in a xe2x80x9cfloatingxe2x80x9d state without any direct connection with external circuits for storing charges. A source voltage VSL is applied to the N+-doped region 16 (source terminal), and a drain voltage VBL is applied to the N+-doped region 18 (drain terminal).
With these applied voltages, electrons (exe2x88x92) eject from the floating gate 26 to the N+-doped region 18 due to the edge Fowler-Nordheim effect and the flash memory cell 10 is programmed. However, upon applying a voltage on the drain terminal, an undesirable depletion region outside the N+-doped region 18 is also produced. Furthermore, hot holes (e+) will be generated leading to hot hole injection in the presence of lateral electric field. These hot holes can severely affect the normal operation of a flash memory cell 10. With a short-circuiting connection between the N+-doped region 18 of the drain terminal and the P-doped region 20, the above-mentioned problems can be prevented. Referring to FIG. 2, a metal contact 30 penetrates through an N+-doped region 32 of each drain terminal and into a P-doped region 34. A bit line voltage VBL is applied to the N+-doped region 32 of each drain terminal through the metal contact 30 so that the N+-doped region 32 and the P-doped region 34 are short-circuited together.
In addition, a predetermined distance 38 between the metal contact 30 and the stacked gate 36 has to be maintained in the conventional flash memory cell for preventing interferences caused by each other. However, increasing cell density is constantly in demand in current market, and such conventional flash memory cell design apparently can not satisfy such demand.
It is therefore a primary objective of the present invention to provide a contactless channel write/erase flash memory cell by varying a connecting mode of a metal contact to increase memory packing density without affecting the source of a neighboring flash memory cell.
It is another object of the present invention to provide a method of fabricating a contactless channel write/erase flash memory cell.
According to the claimed invention, a flash memory array includes a plurality of contactless channel write/erase flash memory cells, and each memory cell includes a multi-level substrate, a first ion doped region, a floating gate, a tunnel oxide layer, a second ion doped region, a third ion doped region, a fourth ion doped region, two isolating oxide layers, a dielectric layer and a control gate. The tunnel oxide is located on the substrate, and the floating gate is located on the tunnel oxide layer, the first ion doped region acting as a drain is located on one side of the floating gate of the substrate, the second ion doped region is located surrounding a bottom of the first ion doped region, the third ion doped region is located beneath the floating gate with one side bordering on the second ion doped region, the fourth ion doped region that acts as a source is located in the substrate with one side bordering on the third ion doped region, the two isolating oxide layers are located on the first ion doped region and the fourth ion doped region respectively, the dielectric layer is located on the floating gate and the two isolating oxide layers, and the control gate is located above the floating gate and the two isolating oxide layers.
According to the present invention, the control gate of the flash memory cell extends laterally in a word line direction, and the first ion doped region and the second ion doped region extend in a bit line direction. Therefore, a metal contact which a bit line voltage applied to can be designed away from any of the first ion doped region and the second ion doped region of the memory cells in a bit line direction to decrease the number of the metal contact and also to reduce the area of the memory array.
The substrate, from bottom to top, includes a N-substrate, a deep P-well and a N-well. The first ion doped region and the fourth ion doped region are N+-doped region formed by implanting phosphorous (P) or arsenic (As) ions, the second ion doped region and the third ion doped region are P-doped region formed by implanting boron (B) ions, and the second ion doped region has a depth much greater than the third ion doped region.
In addition, the first ion doped region and the second ion doped region are short-circuiting together, such as using a metal contact penetrating through junction between the first ion doped region and the second ion doped region, or using a metal contact crossing the exposed first ion doped region and the exposed second ion doped region.
Furthermore, the present invention further provides a fabricating method of a contactless channel write/erase flash memory cell. The flash memory cell is formed on a substrate. First, a shallow P-doped region is formed within the substrate, and then a tunnel oxide layer and a floating gate are formed on the shallow P-doped region, respectively. Next, a deep P-doped region is formed one side of the floating gate in the substrate, and two N+-doped regions are formed on the deep P-doped region and another side of the floating gate within the substrate respectively. Two isolating oxide layers are formed on the two N+-doped regions, and a dielectric layer is formed on the floating gate and the two N+-doped regions. Finally, a control gate is formed on the dielectric layer.
The substrate includes a N-substrate, a deep P-well region and a N-well region. The N-substrate is formed first, and then the deep P-well region is formed on the N-substrate. Finally, an N-well region is formed on the deep P-well region.
At least one bit line metal contact is formed outside the block of the flash memory array. The metal contact penetrates through the isolating oxide layer and the junction between the N+-doped region and the deep P-doped region. In an alternative method, the metal contact crosses the exposed N+-doped region and the exposed deep P-doped region which short-circuits these two regions.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.